Nowadays, high demands are made on the quality of optical and/or technical glass and/or glass-ceramics. On the one hand these glasses are desired to be as homogeneous as possible and free from bubbles and schlieren. On the other, the glasses are desired to be so called “eco glasses” which should, as far as possible, not contain any toxic or environmentally harmful substances, such as arsenic or antimony.
The quality of the final glass and/or the final glass-ceramic is essentially influenced by the quality of refining the glass melt. One approach to improve the quality of refinement is based on the use of high refining temperatures, since an increase in the refining temperature results in a reduction of the viscosity of the melt and thus in an increase of a rising rate of the bubbles in the melt, so that existing or generated bubbles can be better removed from the melt.
Furthermore, for elevated refining temperatures, in particular of more than 1700° C., so called high-temperature refining agents are available. An example of a high-temperature refining agent is SnO2. SnO2 is ecologically harmless, but can only be used at a refining temperature above 1500° C. This allows to omit ecologically questionable refining agents such as As2O5 which can already be used at a temperature above 1250° C.
Refinement in a high-temperature range, in particular above 1700° C., is described in document DE 10 2006 003 521 A1, for example. The melt is heated using electrodes which are placed in the melt. However, the teachings described therein do not only aim to increase the temperature. An essential feature therein is a formation of a stabilized convection roll in the refining vessel which is achieved by generating a large temperature difference in the melt. The temperature difference exists between an inner volume zone of the melt and a peripheral zone of the melt. In this manner, the lateral walls of the refining crucible are cooled. They are cooled to such an extent that the melt solidifies on the cooled lateral walls. So a protective layer of intrinsic material is formed. A so-called “skull crucible” is formed. The basic idea of the described teachings is based on the assumption that in order to form a convection roll it is necessary to cool the melt in the peripheral zones through the cooled lateral walls and at the same time to heat the melt in the interior of the refining vessel using the electrodes. The device shown therein is suitable for producing “eco glasses”. However, the cooling entails very high energy costs. The energy introduced into the melt for heating the melt is “directly” withdrawn from the melt through the cooled lateral walls. Additionally, the power supplies have to be dimensioned appropriately to be able to provide the required electric power. Also, sufficient cooling for the lateral wall has to be provided. Since the cooling of the lateral walls is based on water-carrying copper pipes, the cooling must not fail under any circumstances because this would result in a collapse of the entire system. Therefore, appropriate emergency cooling systems have to be provided, which involves additional complexity and costs.